Merge branch 'master' of github.com:zzhang-cn/dgl into 0.2.x

[Release] update version (dmlc#297)

* update version; add news.md; modify contributing.md

* change urls to dmlc

[Model]Transformer (dmlc#186)

* change the signature of node/edge filter

* upd filter

* Support multi-dimension node feature in SPMV

* push transformer

* remove some experimental settings

* stable version

* hotfix

* upd tutorial

* upd README

* merge

* remove redundency

* remove tqdm

* several changes

* Refactor

* Refactor

* tutorial train

* fixed a bug

* fixed perf issue

* upd

* change dir

* move un-related to contrib

* tutuorial code

* remove redundency

* upd

* upd

* upd

* upd

* improve viz

* universal done

* halt norm

* fixed a bug

* add draw graph

* fixed several bugs

* remove dependency on core

* upd format of README

* trigger

* trigger

* upd viz

* trigger

* add transformer tutorial

* fix tutorial

* fix readme

* small fix on tutorials

* url fix in readme

* fixed func link

* upd